Ruby Regular Expressions
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Transcript Ruby Regular Expressions
Ruby Regular Expressions
AND FINITE AUTOMATA…
Why Learn Regular Expressions?
RegEx are part of many programmer’s tools
vi, grep, PHP, Perl
They provide powerful search (via pattern matching)
capabilities
Simple regex are easy, but more advanced patterns
can be created as needed
From: http://www.websiterepairguy.com/articles/re/12_re.html
Finite Automata
Formally a finite automata is a five-tuple(S, S, ,
s0, SF) where
• S is the set of states, including error state Se. S
must be finite.
• S is the alphabet or character set used by
recognizer. Typically union of edge labels
(transitions between states).
• (s,c) is a function that encodes transitions (i.e.,
character c in S changes to state s in S. )
• s0 is the designated start state
• SF is the set of final states, drawn with double
circle in transition diagram
Simple Example
Finite automata to recognize fee and fie:
S2
e
S0
f
S1
e
i
S4
S3
e
S5
S = {s0, s1, s2, s3, s4, s5, se}
S = {f, e, i}
(s,c) set of transitions shown above
s0 = s0
SF= { s3, s5}
Set of words accepted by a finite automata F forms a language L(F).
Can also be described by regular expressions.
What type of program might need to recognize fee/fie/etc.?
Finite Automata & Regular Expressions
/fee|fie/
/f[ei]e/
e
S2
e
S0
f
S1
i
S4
S3
e
S5
Another Example: Pascal Identifier
/[A-Za-z][A-Za-z0-9]*/
A-Za-z0-9
S0
A-Za-z
S1
Quick Exercise
Create an FSA to recognize dates
Regular Expressions in Ruby
Closely follows syntax of Perl 5
Need to understand
Regexp patterns – how to create a regular expression
Pattern matching – how to use =~
Regexp objects
how to work with regexp in Ruby
Match data and named captures are useful
Handy resource: rubular.com
Defining a Regular Expression
Constructed as
/pattern/
/pattern/options
%r{pattern}
%r{pattern}options
Regexp.new/Regex.compile/Regex.union
Options provide additional info about how pattern
match should be done, for example:
i – ignore case
m – multiline, newline is an ordinary character to match
u,e,s,n – specifies encoding, such as UTF-8 (u)
From: http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html#UJ
Literal characters
/ruby/
/ruby/i
s = "ruby is cool"
if s =~ /ruby/
puts "found ruby"
end
puts s =~ /ruby/
if s =~ /Ruby/i
puts "found ruby - case insensitive"
end
Character classes
/[0-9]/ match digit
/[^0-9]/ match any non-digit
/[aeiou]/ match vowel
/[Rr]uby/ match Ruby or ruby
Special character classes
/./
#match any character except newline
/./m # match any character, multiline
/\d/ # matches digit, equivalent to [0-9]
/\D/ #match non-digit, equivalent to [^0-9]
/\s/ #match whitespace /[ \r\t\n\f]/ \f is form feed
/\S/ # non-whitespace
/\w/ # match single word chars /[A-Za-z0-9_]/
/\W/ # non-word characters
NOTE: must escape any special characters used to create
patterns, such as . \ + etc.
Repetition
+ matches one or more occurrences of preceding
expression
e.g., /[0-9]+/ matches “1” “11” or “1234” but not empty
string
? matches zero or one occurrence of preceding
expression
e.g., /-?[0-9]+/ matches signed number with optional
leading minus sign
* matches zero or more copies of preceding
expression
e.g., /yes[!]*/ matches “yes” “yes!” “yes!!” etc.
More Repetition
/\d{3}/ # matches 3 digits
/\d{3,}/ # matches 3 or more digits
/\d{3,5}/ # matches 3, 4 or 5 digits
Non-greedy Repetition
Assume s = <ruby>perl>
/<.*>/ # greedy repetition, matches <ruby>perl>
/<.*?>/ # non-greedy, matches <ruby>
Where might you want to use non-greedy repetition?
Grouping
/\D\d+/ # matches a1111
/(\D\d)+/ # matches a1b2a3…
/([Rr]uby(, )?)+/
Would this recognize:
“Ruby”
“Ruby ruby”
“Ruby and ruby”
“RUBY”
Alternatives
/cow|pig|sheep/ # match cow or pig or sheep
Anchors – location of exp
/^Ruby/ # Ruby at start of line
/Ruby$/ # Ruby at end of line
/\ARuby/ # Ruby at start of line
/Ruby\Z/ # Ruby at end of line
/\bRuby\b/ # Matches Ruby at word boundary
Using \A and \Z are preferred
Pattern Matching
=~ is pattern match operator
string =~ pattern OR
pattern =~ string
Returns the index of the first match or nil
puts "value: 30" =~ /\d+/ => 7 (location of 30)
# nil doesn’t show when printing, but try:
found = "value: abc" =~ /\d+/
if (found == nil)
puts "not found"
end
Regexp class
Can create regular expressions using Regexp.new or
Regexp.compile (synonymous)
ruby_pattern = Regexp.new("ruby",
Regexp::IGNORECASE)
puts ruby_pattern.match("I love Ruby!")
=> Ruby
puts ruby_pattern =~ "I love Ruby!“
=> 7
Regexp Union
Creates patterns that match any word in a list
lang_pattern = Regexp.union("Ruby", "Perl", /Java(Script)?/)
puts lang_pattern.match("I know JavaScript")
=>
JavaScript
Automatically escapes as needed
pattern = Regexp.union("()","[]","{}")
MatchData
After a successful match, a MatchData object is
created. Accessed as $~.
Example:
"I love petting cats and dogs" =~ /cats/
puts "full string: #{$~.string}"
puts "match: #{$~.to_s}"
puts "pre: #{$~.pre_match}"
puts "post: #{$~.post_match}"
Named Captures
str = "Ruby 1.9"
if /(?<lang>\w+) (?<ver>\d+\.(\d+)+)/ =~ str
puts lang
puts ver
end
Read more:
http://blog.bignerdranch.com/1575-refactoring-regular-
expressions-with-ruby-1-9-named-captures/
http://www.ruby-doc.org/core-1.9.3/Regexp.html (look for
Capturing)
Creating a Regular Expression
Complex regular expressions can be difficult
Finite automata are equivalent to regular
expressions (language theory)
Quick Exercise
Create regex for the following. Use rubular.com to
check it out.
Phone numbers
(303) 555-2222
303.555.2222
3035552222
Date
nn-nn-nn
Try some other options
Some Resources
http://www.bluebox.net/about/blog/2013/02/using
regular-expressions-in-ruby-part-1-of-3/
http://www.ruby-doc.org/core-2.0.0/Regexp.html
http://rubular.com/
http://coding.smashingmagazine.com/2009/06/01/essent
ial-guide-to-regular-expressions-tools-tutorials-andresources/
http://www.ralfebert.de/archive/ruby/regex_cheat_sheet/
http://stackoverflow.com/questions/577653/differencebetween-a-z-and-in-ruby-regular-expressions (thanks, Austin
and Santi)
Topic Exploration
http://www.codinghorror.com/blog/2005/02/regex-use-vs-regex-abuse.html
http://programmers.stackexchange.com/questions/113237/when-you-should
not-use-regular-expressions
http://coding.smashingmagazine.com/2009/05/06/introduction-toadvanced-regular-expressions/
http://stackoverflow.com/questions/5413165/ruby-generating-new-regexpsfrom-strings
A little more motivation to use…
http://blog.stevenlevithan.com/archives/10-reasons-to-learn-and-use-regularexpressions
http://www.websiterepairguy.com/articles/re/12_re.html
Submit on BB (3 points) and report back: 3-5 things you want to remember
about regex. Include the URL. Feel free to read others not in the list.
This is an individual exercise.